Control method and apparatus for screwing down reeling rolls

ABSTRACT

A control method for controlling the screwing down pressure provided in a reeling mill where the reels are controlled by a combination of signals continuously monitored by a computer. The signals include the temperature of the pipe entering the reeling mill, the voltage and current supplied to the reeling motor and the speed of the reeling motor. A calculation is then performed which gives the actual reduction of wall thickness based upon a predetermined relationship of the temperature of the pipe entering the reeling mill and the torque of the rolling motor. The torque is calculated based upon the speed, current and voltage of the rolling motor. The pressure supplied to the rolling reels is then altered based on this signal.

FIELD OF THE INVENTION

This invention relates to a method for automatic control of thescrewing-down of reeling rolls in a reeling mill. More particularly, theinvention relates to a method for automatically controlling thescrewing-down of rolls in the process of reeling in the production ofseamless steel pipes by the Mannesman plug mill method.

BACKGROUND OF THE INVENTION

The reeling mill (reeler) is layed between the plug mill and sizing millin the process for manufacture of seamless steel pipes. In this mill,the pipe is "reeled" by reducing its wall thickness for various purposesamong which are the correction of the thickness deviation that hasdeveloped in the previous rolling mills (i.e. piercer, elongator andplug mill), elimination of the flaws developed by abrasion with the plugin the plug mill rolling, and ensuring a suitable amount of pipeexpansion for the sizing mill in the subsequent step. It is the mostimportant feature of the reeling mill that the greater part of theamount of reduction in wall thickness changes into the expansion of theoutside diameter of the pipe, so if a variation occurs in the amount ofreduction of the wall thickness during reeling, the outside diameter ofthe reeled pipe also changes in its longitudinal direction and thefollowing disadvantages result: if the amount of reduction in wallthickness during reeling is small, the degree of increase in the outsidediameter is small and the desired outside diameter of the pipe cannot beachieved, and vice versa. If the outside diameter of the reeled pipe issmaller than a predetermined value, not all parts of the pipe is rolledin the subsequent sizing mill, which has a very adverse effect on theoutside diameter of the final product. If the outside diameter of thereeled pipe is larger than the predetermined value, a flaw such as onedue to the roll edges develops during rolling in the sizing mill, whichis also detrimental to the quality of the final product. So, theoperation in the reeling mill has great effect on the accuracy of thedimensions of the final product, hence its yield, and it is mandatory inthe reeling mill to roll the pipe to have the desired outside diameter.

Several methods are known for controlling the operation of reelingmills. One of them is described in Japanese Patent Application (OPI) No.37568/78 (the symbol OPI as used herein means an unexamined publishedJapanese Patent Application), and to provide a pipe of constant outsidediameter on the leaving side of the mill, the electric power for rollingis kept constant for each lot and for the entire length of each pipe tobe rolled. Another method is described in Japanese Patent Application(OPI) No. 86663/78, and to provide a pipe of constant wall thickness onthe leaving side, the pattern of electrical power outputs is determinedfor each pipe on the basis of information of the cross section andtemperature of the pipe on the entry side so that the cross section ofthe pipe on the leaving side is kept constant for each lot and for theentire length of each pipe, and the electric power for rolling ischanged after this pattern.

The method of Japanese Patent Application (OPI) No. 37568/78 disregardsthe change in the temperature of each pipe to be rolled as well as thechange in the temperature in the longitudinal direction of the pipe. So,if the reeling operation is performed with the electric power heldconstant, the reduction in the wall thickness at the low temperatureportion of pipe where occurs the larger resistance to deformation is sosmall that the pipe on the leaving side cannot be expanded to thepredetermined value of outside diameter. What is more, the outsidediameter of the pipe on the entry side of the mill is also varied inlongitudinal direction by the rolling force and by the set of roll gapin the plug mill rolling, so this adds to the variation in the outsidediameter of the pipe that is leaving the reeling mill under the controlof constant electric power.

The method of Japanese Patent Application (OPI) No. 86663/78 achievesits object by changing the desired electric power according to apredetermined pattern by taking into account the change in thetemperature of the pipe in its longitudinal direction, but as in thefirst method, it disregards the change in the outside diameter of thepipe on the entry side in defining the target value for the electricpower that provides a pipe of constant cross-sectional area on theleaving side of the mill. As a result, there occurs a variation in theoutside diameter of the reeled pipe both in the longitudinal directionand with respect to the average outside diameter of the pipes for eachlot. The second method has another defect: the pipe to be fed into theplug mill generally has such a temperature distribution in thelongitudinal direction that the temperature increases from the leadingend to the trailing end, and so the wall thickness of the pipe leavingthe plug mill often increases from the leading end to the trailing end.Therefore, the pipe entering the reeling mill generally has a crosssection that decreases from the loading end to the trailing end.

To reel the pipe having a cross-sectional area of the pattern describedabove and provide a product whose cross section is uniform in thelongitudinal direction, the amount of reduction in wall thickness mustbe decreased from the leading to trailing edge, but then, the resultingpipe on the leaving side has an outside diameter that decreases from theleading to trailing end and which is not desired in the final product.Still another defect that is common to the two methods is that in spiteof the supply of a constant electric power for rolling, a change in thespeed of the rolling motor during reeling causes change in the rollingtorque, and as a result, the desired outside diameter is not attained.

Therefore, one object of this invention is to provide a control methodfor screwing down the reeling rolls that is free from the abovedescribed defects of the conventional techniques and which keepsproviding a pipe of constant outside diameter on the leaving side of thereeling mill by first determining the desired amount of reduction inwall thickness in the longitudinal direction of the pipe being reeled,calculating the actual amount of reduction in wall thickness from thetorque of the rolling motor and the temperature of the pipe on the entryside and controlling said actual amount of reduction in wall thicknessto be equal to the desired amount of reduction in wall thickness.

Another object of this invention is to provide a control method forscrewing down the reeling rolls that achieves quantitative adjustment ofthe roll gap so that the pipe leaving the reeling mill has a uniformoutside diameter in the longitudinal direction.

Still another object of this invention is to provide an apparatus thatis used with advantage in performing the above described controlmethods.

DESCRIPTION OF THE INVENTION

According to this invention, a control method for screwing down reelingrolls is provided that keeps providing a pipe of constant outsidediameter on the leaving side of the reeling mill. To achieve thispurpose, the desired amount of reduction in the wall thickness of thepipe being reeled is determined for the longitudinal direction of thepipe from the outside diamter and average wall thickness of the pipe onthe entry side of the reeling mill and the desired outside diameter ofthe pipe on the leaving side, the actual amount of reduction in the wallthickness of the pipe being reeled is calculated for the longitudinaldirection of the pipe from the temperature of the pipe on the entry sideof the reeling mill and the rolling torque of a rolling motor, and thedegree of screwing down the reeling rolls is controlled so that theactual amount of reduction in wall thickness is equal to said desiredamount of reduction in wall thickness.

This invention also provides a control apparatus for screwing downreeling rolls which includes a wall thickness and OD arithmetic meansfor calculating the average wall thickness and the outside diameter of apipe being fed into the reeling rolls, an AGC means which receivesinformation on said average wall thickness and outside diameter of thepipe on the entry side of the reeling mill, the temperature of the pipeon the entry side of the reeling mill, the voltage and current appliedto the armature of a rolling motor, the speed of rotation of the motor,and the desired outside diameter of the pipe on the leaving side of themill, and calculates the desired amount of reduction in wall thicknessand the actual amount of reduction in the wall thickness of the pipebeing rolled to thereby deliver a signal indicative of the differencebetween the desired amount of reduction in wall thickness and the actualamount of reduction in wall thickness, and a screw down motor controlmeans that delivers a screw down signal to a screw down motor inresponse to the difference signal. The AGC means as used herein means an"automatic roll gap control means" which delivers a signal indicative ofthe difference between the desired amount of reduction in wall thicknessand the actual amount of reduction in wall thickness to the screw downmotor control means for driving the motor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the control system of this inventionas used in a reeling mill;

FIG. 2 shows schematically a cross section of a pipe being rolled in thereeling mill; and

FIG. 3 shows schematically a cross section of a pipe being rolled in aplug mill prior to the reeling mill.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One preferred embodiment of this invention is now described specificallyby reference to the accompanying drawings. FIG. 1 is a block diagramshowing the control system of this invention as used in a reeling mill.FIG. 2 is a schematic representation of a cross section of a pipe beingrolled in the reeling mill. As shown, a pipe 3 is rolled by a pair ofbarrel-shaped reeling rolls 6,6 (the longitudinal axes of which crosseach other) and a reeling plug 7. As the rolls and plug rotate to thedirections indicated by the arrows, the reduction in the wall thicknessis performed. As a result, the thickness deviation and flaws such asabrasions caused by the plug in the preceding rolling are eliminated. Inthe Figures, 1 is a guide shoe that regulates the vertical position ofthe pipe 3,8 is a plug bar of the plug 7,9 is a screw attached to theshafts of the rolls 6,6 for controlling the roll gap, 10 is a screw downmotor for operating the screw 9, and 12 is a rolling motor that rotatesthe rolls 6,6. In the reeling step, the greater part of the amount ofreduction in wall thickness is converted into the expansion of theoutside diameter of the pipe, so it is necessary to control the amountof reduction in wall thickness for the purpose of controlling theoutside diameter of the pipe that leaves the reeling mill.

To know the relation between the amount of reduction in wall thicknessand the expansion of the outside diameter of the pipe, we have analyzedmany data that were obtained by our reeling operations, and have foundthat the desired amount of reduction in wall thickness (Δt_(o)) can bedetermined by the following formula (1): ##EQU1## Δt_(o) : the desiredamount of reduction in wall thickness; D_(R) : the desired outsidediameter of the reeled pipe (constant in longitudinal direction);

D_(P) : the actual outside diameter of pipe leaving the plug mill (ODpattern in longitudinal direction); and

t_(p) : the average wall thickness of the pipe leaving the plug mill.

In the formula, the value of α is determined for the dimensions of aspecific pipe and the kind of the steel. By using this formula one candetermine optimum amount of screwing down the reeling rolls forattaining the desired outside diameter of the leaving pipe (D_(R)) evenif the outside diameter of the incoming pipe (D_(P)) varies in thelongitudinal direction of the pipe.

The method of determining the outside diameter of the pipe leaving theplug mill (D_(p)) and its average wall thickness (t_(p)) is describedbelow.

The outside diameter at various points in the longitudinal direction ofthe pipe leaving the plug mill is calculated by the following formula(2) from the shape of the caliber formed by upper and lower rolls in theplug mill (as indicated by 2 and 5 in FIG. 3), the roll gap G, and therolling load P for the operation of the plug mill. The formula (3) canbe easily obtained from FIG. 3 wherein a rolling plug is indicated by 4.##EQU2## wherein G_(o) is a reference roll gap, M is the mill rigidityof the plug mill, r₁ is the radius of the bottom of the caliber, r₂ isthe radius of the flange of the caliber, θ₁ is the angle of a sectorhaving a radius r₁, and θ₂ is the angle of a sector having a radius r₂.

The rolling load P and roll gap G vary not only for each pipe but alsoin the longitudinal direction of the same pipe being rolled, so they arethe factors that cause a change in the outside diameter in longitudinaldirection of the pipe leaving the plug mill. The leading and trailinglow-temperature ends of the pipe being fed into the plug mill are placedunder a rolling load significantly higher than the one applied to themiddle of the pipe, so the difference between the outside diameter ofthe leading and trailing ends and that of the middle portion is notnegligible.

The outside diameter of the pipe leaving the plug mill can be obtaineddirectly without using the formula (2). In the direct method, theoutside diameter of the pipe leaving the plug mill or coming into thereeling mill is measured continuously in the longitudinal direction byan OD measuring instrument. The average outside diameter of the pipeleaving the plug mill (D_(p)) that is mentioned hereunder is determinedby averaging the respective values of the outside diameter measured atvarious points of the pipe in the longitudinal direction. The data ofD_(p) is fed into the AGC means 15 (FIG. 1) as information on theoutside diameter of the pipe on the entry side of the reeling mill.

The average wall thickness (t_(p)) of the pipe leaving the plug mill isdetermined by the following procedure. The weight of a billet ismeasured before it is charged into the heating furnace, the weight ofthe pipe in the plug mill (W) is determined by subtracting the scaleloss in the heating furnace, etc., and the length of the pipe (l_(p))that has been rolled for the final pass in the plug mill is measureddirectly. The value of t_(p) is calculated by the following formula (3)from the measurements of W, l_(p) and D_(p) : ##EQU3## wherein ρ_(p) isthe density of the steel and depends on the temperature of the steel.The data on the average wall thickness (t_(p)) is fed into the AGC means15 as information on the wall thickness of the pipe on the entry side ofthe reeling mill.

On the basis of the formulas (1), (2) and (3) described above, one candetermine the optimum (desired) amount of reduction in wall thickness(Δt_(o)) for both a specific pipe and its longitudinal direction that isnecessary to attain the desired outside diameter D_(R) of the pipe onthe leaving side of the reeling mill. Alternatively, the wall thickness(t'_(p)) may be directly determined by a pipe thickness measuringinstrument as in the case of direct measurement of the outside diameterof the pipe. For this purpose, a known hot wall thickness gauge isinstalled on the leaving side of the plug mill, and the respectivevalues of the wall thickness of the pipe leaving the plug mill that areobtained by continuous measurements are averaged to calculate the t_(p).

The actual amount of reduction in wall thickness (Δt_(A)) is determinedfrom the electric power of the rolling motor in the reeling mill and thetemperature of the pipe on the entry side of the reeling mill by takinginto consideration the resistance to deformation. We have discovered thefollowing formula (4) that represents the relation between the rollingtorque of the rolling motor and the amount of reduction in wallthickness. The formula assumes the use of a d.c. motor. ##EQU4## V_(a) :the voltage on the armature of the rolling motor, I_(a) : the current onthe armature of the rolling motor,

N_(M) : the motor speed,

K_(f) : resistance to deformation, D_(R) : the desired OD of the pipeleaving the reeling mill, β,γ: constants. The resistance to deformation(K_(f)) is determined from the temperature of the pipe being rolled (T),carbon content of steel, etc. Many formulas have been proposed forcalculation of K_(f) and a suitable one may be selected depending uponthe need. The temperature of the entry pipe being rolled is measured onthe entry side of the reeling mill (T) with a thermometer 13. The exittemperature is measured on the exit side of the reeling mill bythermometer 14. Needless to say, K_(f) varies in the course of rollingaccording to the temperature pattern in the longitudinal direction ofthe pipe. The data on the factors described above are fed into the AGCmeans 15 of FIG. 1 which calculates the actual amount of reduction inwall thickness Δt_(A) by the formula (4) on the basis of these data andthose on I_(a), V_(a), N_(M) and T which are also fed to the AGC. The sodetermined actual amount of reduction in wall thickness Δt_(A) issubstracted from the desired amount of reduction in wall thicknessΔt_(o), and the position to which the reeling rolls are screwed down isadjusted according to the difference. The roll gap E is adjusted to E-2(Δt_(o) -Δt_(A)) by the screw down motor control means 11 (FIG. 1): whenΔt_(A) is smaller than Δt_(o), the gap is reduced to increase theΔt_(A), and if Δt_(A) is larger than Δt_(o), the roll gap is increasedto decrease the Δt_(A).

As described above, this invention provides a method for quantitativeadjustment of the roll gap that has been impossible in the conventionaltechnique.

Probability Of Industrial Utility

The prior art technique controls the screwing down of reeling rollswithout taking into consideration the change in the temperature inlongitudinal direction of the pipe on the entry side of the reelingmill, so the pipe leaving the rolls does not have a uniform outsidediameter in the longitudinal direction, but this problem is notencountered in the method of this invention. In particular, a pipe bothends of which have an outside diameter equal to the desired value can beproduced by the method of this invention.

Some of the prior art techniques provide a reeled pipe with a constantwall thickness only by scarificing the uniformity of the outsidediameter of the pipe in its longitudinal direction, but this problem isalso eliminated from the method of this invention.

The prior art technique is not capable of quantitative control of thedegree by which the reeling rolls are screwed down, so depending on thedimensions of the pipe to be rolled, hunting or delay effect frequentlyoccurs. According to this invention, the torque of the rolling motor ismeasured continuously to detect the actual amount of reduction in thewall thickness of the pipe at particular points of time, and so, onlyone value is determined for the amount of correction of the gap betweenthe two rolls when the actual amount of reduction in wall thicknessdeviates from the desired amount of reduction in wall thickness. Thisachieves very reliable control operation without hunting or delayeffect.

As described in the foregoing, the method of this invention assures theproduction of a seamless steel pipe of good quality whose outsidediameter is equal to the desired value and which is entirely free fromany flaw such as one due to the roll edges that develops during rollingin the sizing mill.

What is claimed is:
 1. A method for controlling the automatic rolling of pipes in a reeling mill comprising the steps of:determining the desired amount of reduction in wall thickness of said pipe being reeled; continuously monitoring the voltage and current supplied to the rolling motor; continuously monitoring the speed of said rolling motor; calculating the torque of said rolling motor by a predetermined mathematical relationship of the voltage, the current and the speed of said rolling motor; measuring the temperature of said pipe on the entry side of said rolling mill; calculating the actual reduction in wall thickness based upon a predetermined relationship of said temperatures and said torque; generating correction signals based upon the relationship between the desired reduction and said calculated actual reduction; and adjusting the screwing down of the reeling rolls in response to said correction signal.
 2. A method according to claim 1 further comprising the steps of:directly measuring the outside diameter of the pipe supplied to said reeling mill; directly measuring the wall thickness of said pipe supplied to said reeling mill. 